Charge coupled device (CCD) image sensors are well-known in the art and are discussed in Sequin et al, Charge Transfer Devices, Academic Press, New York (1975), pages 142-200. Such charge coupled imagers include focal plane arrays of semiconductive photodetectors formed on the surface of a semiconductor substrate. The detectors are arranged in columns, each column of detectors adjacent and parallel its own CCD column register, into which each of the detectors in the column feeds charge. The focal plane array includes a plurality of parallel columns of photodetectors and a corresponding plurality of CCD column registers. The plurality of column registers transfers charge from the plurality of photodetectors into a multiplexing register disposed at the top of the photodetector columns and perpendicular thereto. An optical system focuses an image onto the focal plane array. Simultaneously, charge transfer occurs in the plurality of column registers and in the multiplexing register so that the charge packets dumped out of the multiplexing register correspond to real time serial data analogous to a television signal. The field of view may be increased and the signal-to-noise ratio improved by having the optical system scan an image across the focal plane array in a direction parallel to the columns of photodetectors and toward the multiplexing register in synchronism with the charge transfer in the readout column registers. It is well-known to those skilled in the art that this type of scanning results in time delay and integration of stationary images in each of the column readout registers.
A significant problem which limits performance of focal plane arrays is that the responsivities of individual photodetectors in the array are typically non-uniform. The term "responsivity" refers to the ratio of the amount of current generated in a semiconductor photodetector divided by the power of the beam of photons incident on the detector, which ratio is a constant characteristic of the physical properties of an individual detector. If the non-uniformity in responsivity is sufficient, the average responsivity of each of the columns of photodectors may differ widely from column to column, distorting the serial data dumped from the multiplexing register. Although it is possible to make substantial compensation for such distortion using conventional techniques, such conventional compensation requires additional hardware provided externally of the focal plane array substrate, including hardware to demultiplex the output signal dumped from the multiplexer register and to provide timing and memory functions. Such additional hardware consumes space and power, a significant disadvantage.